1. Field of the Invention
This invention relates to a method for forming a metal film, and more particularly to a method for forming a metal film with twins.
2. Description of the Related Art
It is well known to persons having ordinary skill in the art of metallurgy that, after a metal material is applied with a shear stress through a cold working procedure, line defects in the metal material, such as dislocations, may be moved by virtue of slip system in the metal material itself. The moved dislocations may become entangled with one another. The application of cold working also increases planar defects in the metal material, such as twins. The tangled dislocations and the planar defects enhance mechanical strength of the metal material, such as hardness. However, the abovementioned cold working procedure is limited for application only to bulks, and is not suitable for a thin metal film.
In recent years, to enhance the strength of a metal film in order to expand the applicability of the metal film, growth twins have attracted much attention in the field of thin film processing. It has been recognized that the smaller the twin spacing of growth twins, the greater the mechanical strength will be. The theory and mechanism behind this result are similar to those of grain size strengthening. That is, fineness of grains facilitates enhancement of the mechanical strength of the metal material.
In an article entitled “High-strength sputter-deposited copper foils with preferred orientation of nanoscale growth twins” in APPLIED PHYSICS LETTERS 88, 173116 (2006), X. Zang et al disclosed magnetron sputtering copper at a deposition rate of 0.5 nm/s to 2.0 nm/s to produce a 20 μm copper foil on a Si (100) substrate with a native oxide layer. No heating or cooling was applied to the Si substrate during the formation of the copper foil.
Analytic results of the copper foil using transmission electron microscope (TEM) indicate that the copper foil sputter-deposited at a rate of 1.8 nm/s has columnar grains with an average size of 43 nm. The TEM results also show an extremely high density of planar defects within the columnar grains. Also, analytic results of the copper foil using high resolution transmission electron microscope (HRTEM) indicate that the planar defects are growth twins with {111} interfaces. The spacing between two adjacent twins is approximately 5 nm, and the planar defects of {111} twin interfaces are stacked along the growth direction of the copper foil. This confirms that the planar defects are stacking faults (SF) within the copper foil. In other words, the twins in the copper foil resulted from the stacking faults of the close-packed plane of the copper foil.
In addition, five uniaxial tensile tests were performed on the copper foils. The results indicate that the average elastic modulus, the average tensile strength, and the average yield strength of the copper foils are approximately 110 GPa, 1.2 GPa and 1.1 GPa, respectively. Further, the hardness of the copper foil, as measured by a nanoindenter, was 3.5 GPa.
In an article entitled “Microstructural stability during cyclic loading of multilayer copper/copper samples with nanoscale twinning” in Scripta Materialia (2009) 1073-1077, C. J. Shute et al disclosed sputtering of copper using magnetron sputtering deposition technique to produce a 178 μm copper multilayer film on a Si (100) substrate. The thickness of the copper multilayer film is sufficient to allow the copper multilayer film to be removed from the Si (100) substrate.
The copper multilayer film has a mirror-like surface that is in contact with the Si (100) substrate, and a dull surface that is opposite to the mirror-like surface. A Vickers hardness measurement was conducted on the copper multilayer film. The result indicates that the hardness of the mirror-like surface and the dull surface are 1.1 GPa and 1.9 GPa, respectively. In addition, an analysis of the copper multilayer film was performed by means of a focus ion beam (FIB) microscope. The results indicate that the structure of the copper multilayer film changes from a non-columnar microstructure to a nanotwinned columnar microstructure from the mirror-like surface to the dull surface.
From the above, it is found that, when a metal film with twins are formed using magnetron sputtering deposition technique, a predetermined deposition thickness is required to convert the microstructure of the metal film into a twin structure. For a copper wire that is widely used in a semiconductor device, the thickness of the copper wire is required to be in the range from about 300 nm to about 400 nm. Therefore, the methods for forming a metal film with twins disclosed by C. J. Shute et al and C. Zang et al are not suitable for the nano-scale copper wire that is used in the semiconductor field.